Wintry DoomVisions of climate catastrophe drew upon a widespread fear that
nuclear war could wreck the global environment. Scientific calculations,
publicized in 1983, suggested that the bombs could pollute the air with
enough dust and chemical smog to severely cool the planet  a "nuclear
winter." The lesson about the atmosphere's fragility was meanwhile
reinforced by evidence that such a climate catastrophe had actually happened
long ago. Something, perhaps a single asteroid-bomb, had caused a global
cooling that exterminated the dinosaurs.

In the 1950s, as the world's arsenals filled with hydrogen bombs,
people worried about how a thermonuclear war might injure the entire
global environment. Poignant novels and movies showed radioactive
dust, borne on the winds, extinguishing all life on Earth.(1) Experts dismissed the scenarios as impossible.
But secret studies supported by the U.S. military suggested that a
war's effects on the atmosphere could be quite serious. In an openly
published, but little noticed, 1958 review of climatology, a leading
expert wrote that a nuclear war could throw up enough dust to alter
the climate for a few years.(2*)

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That remained the
received view among experts. A National Academy of Sciences study
that reviewed the issue in 1975 concluded that a war could kick up
as much dust and smoke as a large volcanic eruption. Scientists suspected
that such eruptions in the past had cooled the Earth a degree or so
for a year or two. Moreover, recent spacecraft observations of Mars
had showed that a planet-wide dust storm could become self-sustaining.
Still, the authors concluded that the effects "would probably lie
within normal global climatic variability," and it seemed like a minor
problem next to the other horrors of a nuclear war. Meanwhile, the
nitrogen compounds (NOx)
created by the fireballs could sharply reduce the Earth's ozone layer,
but again the damage would be only temporary. The authors did admit
that little was known about climate, so that "the possibility of climatic
changes of a more dramatic nature cannot be ruled out." A few scientists
criticized the report for brushing aside possible calamities. Some
pointed to the vast firestorms that a war could ignite, exclaiming
that might pollute the atmosphere so severely as to "force Homo
sapiens into extinction."(3) These scientists were in tune with a public
attitude that grew strong during the 1960s and 1970s. For the first
time, many people found it plausible that we could bring about an
atmospheric catastrophe so terrible that it would destroy the human
race.

This commonly held attitude may have helped
scientists to admit into their thinking a new answer to an old puzzle.
Geologist Walter Alvarez and his physicist father, Luis Alvarez, proposed
that the fall of a huge asteroid had caused the extinction of the
dinosaurs some 65 million years ago. They figured that the dust thrown
into the air by an impact could have obscured sunlight long enough
to kill much of the Earth's plant life through simple darkness, so
that the dinosaurs perished of starvation. Stephen Schneider recalled
that when he heard Luis Alvarez explain the new idea in a 1979 lecture,
"I commented from the floor that such a cloud could have climatic
effects, particularly a sharp, but short-term climatic cooling on
land." Schneider was just then involved in studies that emphasized
how smoke, smog, and other aerosols could cool the atmosphere and
perhaps even precipitate an ice age. Calculations soon confirmed that
an asteroid strike could indeed have brought on a global cooling severe
enough to kill off the dinosaurs directly.(4)

Other scientists scoffed at the idea, especially geologists and
paleontologists who stuck to their old theories about dinosaurs. These
theories, however, failed to fit observations of world-wide peculiarities
preserved in rock layers 65 million years old. Some geologists proposed
that the damage to the atmosphere had not been due to an asteroid
strike, but to CO2 and other gases from an enormous,
"paroxysmal" spate of volcanic eruptions. There was evidence of just
such a volcanic outpouring at about the right time. Either way, the
killer had been a shocking atmospheric change.(5)

The dinosaur-extinction
debate became passionate, sometimes personal and embittered, carrying
forward a tradition of geological controversy that stretched back
to the 18th century. On one side had been traditional "catastrophists,"
whose historical roots connected them with Bible fundamentalists
and Noah's Flood. They had argued ardently that vast cataclysms
in the past had suddenly extinguished entire sets of species. By
the late 19th century these views had been driven from the field
of professional scientific discussion by the views of so-called
"uniformitarians" (a more precise term would have been "gradualists.")
These scientists had amassed convincing evidence that evolution
acted over millions of years, responding to the slow rise of mountain
chains or the parting of continents.

By 1980, however, some paleontologists were beginning to be persuaded
that species could evolve in a "punctuated" pattern. In short, the
catastrophist viewpoint was raising its head again. Anyway that
was how the opponents caricatured the movement  the actual
scientific arguments were of course more complex.(6) Underneath the science,
what mattered was a picture in which dinosaurs did not decline gradually
over eons, but fell in their prime, struck down by a random doom.
The unspoken and repugnant implication was that any species (maybe
even our own) could be extinguished in an arbitrary moment.

The most likely way that could happen was
through nuclear war. The effect of bombs on climate had been taken
up again in 1981 by Paul Crutzen. A Dutch scientist interested in
aerosols, Crutzen had helped set off the stratospheric transport controversy
of the early 1970s (described here)
by showing how airplane emissions could destroy ozone. After working
at the Air Quality Division of the U.S. National Center for Atmospheric
Research, he was now employed in Germany. Crutzen had recently been
in Brazil, collecting samples of smoke to check the contentious claim
that slash-and-burn destruction of forests was a major source of atmospheric
CO2. Reviewing the 1975 National Academy report,
Crutzen worried that the study group had focused on dust without taking
full account of how much smoke, NOx,
and other smog could arise from the firestorms of industrial centers
and forests torched by bombs. People had known for many decades that
the smoke from great forest fires could dim the sunlight thousands
of miles downwind. Crutzen concluded that nuclear war, much like the
Alvarez asteroid, could send the world into a frozen twilight.(7)

Atmospheric scientists were well-placed to
take up the question of smoke from a nuclear war. Measurements like
Crutzen's of the effects of soot and the like had greatly advanced
since the 1975 study. Richard Turco and others, working on the dinosaur
extinction problem, had developed a computer model of a haze-filled
atmosphere, and it had occurred to them that dust lofted by the explosions
of a nuclear world war might have effects comparable to the dust from
an asteroid impact. Meanwhile James Pollack and Brian Toon had been
working with Carl Sagan on how the aerosol smoke from volcanoes could
affect climate. Joining forces, the groups calculated that after an
exchange of hydrogen bombs, the sooty smoke from burning cities could
bring on a "nuclear winter"  months or even years of cold so
severe it would gravely endanger living creatures.(8)

The scientists did this work mainly for public consumption. When they announced
their results in 1983, it was with the explicit aim of promoting international
arms control. Surely the likelihood that all-out nuclear war was literally
suicidal would persuade nations to reduce their arsenals? As a side
effect, the studies helped to improve scientific understanding of
how aerosols could affect climate.(9)

The computer models were so simplified, and the data on smoke and
other aerosols were still so poor, that the scientists could say nothing
for certain. Critics, mostly people opposed to nuclear disarmament,
quickly pointed out the deficiencies. In the mid 1980s, detailed studies
confirmed that a nuclear war would probably alter global climate temporarily.
But as Schneider and a coauthor explained in a widely read article,
it was not likely to bring an apocalyptic winter, but only a dangerous
"nuclear fall."(10) (More recent research has not changed the situation: the
horrors of full-scale nuclear war would probably, but not certainly,
include a severe but temporary degradation of climate.) There were
so many variable factors that nobody could say with confidence what
would happen.

By the late 1980s, a wide variety of geological
evidence supported the hypothesis that the doom of the dinosaurs had
been a climate catastrophe, caused by a great asteroid strike. The
cause perhaps included enormous volcanic outbursts, and certainly
a great asteroid strike, which had shrouded the atmosphere not only
with dust but with smoke from vast wildfires ignited by the asteroid’s
blazing descent. Accepting the idea, most geologists moved on to inquire
whether exceptional asteroids or volcanic eruptions might have caused
the other great extinctions in the geological record. The vociferous
disputes over nuclear winter and dinosaur extinction had made scientists
and the public more sensitive than ever to the way stuff emitted into
the air could push a severe climate change.